Heavenfall's amazing work on the Rivermod inspired to me see if I could use a stamp to unlock and summon the Darkling cityhub type that Deorcnysse, in the Shroud Wildlands, uses. After some tinkering, it worked.

This is the stamp:

<StampTypes>
    <DataChecksum NoParse="1">
        <Ignore>DisplayName</Ignore>
        <Translate>DisplayName</Translate>
    </DataChecksum>
    <StampType InternalName="DeorcnysseStamp">
        <DisplayName>Deorcnysse</DisplayName>
        <IsCore>0</IsCore>
        <Frequency>0</Frequency>
        <Style>0</Style>
        <Rules>1</Rules>
        <TileWidth>1</TileWidth>
        <TileHeight>1</TileHeight>
        <MapTile InternalName="volcano_Tile_0_0">
            <LocalX>0</LocalX>
            <LocalY>0</LocalY>
            <TerrainType>Land</TerrainType>
            <Environment>0</Environment>
            <Height>51.564</Height>
            <ContinentID>1741</ContinentID>
            <GeographicalID>0</GeographicalID>
            <GroupFlag>0</GroupFlag>
            <DistanceFromSpawnPoint>0.0000</DistanceFromSpawnPoint>
            <DirectionToSpawnPoint>8</DirectionToSpawnPoint>
            <NameIndex>-1</NameIndex>
            <ExtraData>-1</ExtraData>
            <Mark>0</Mark>
            <HasRoad>0</HasRoad>
            <AdjacentRoadTile>0</AdjacentRoadTile>
        </MapTile>
        <GameObjectCreationParams InternalName="W_Shroud_01_Improvement_1.0000,1.0000,51.5644">
            <DisplayName>Deorcnysse</DisplayName>
            <Position>1.0000,1.0000,51.5644</Position>
            <ObjectType>Improvement</ObjectType>
            <DataTypeName>DarklingCityHub1</DataTypeName>
            <Facing>0</Facing>
            <SpawnOption>0</SpawnOption>
            <DesiredSpawnRating>-1</DesiredSpawnRating>
            <WanderRadius>-1</WanderRadius>
            <Owner>0</Owner>
            <SpawnCount>1</SpawnCount>
            <Level>1</Level>
        </GameObjectCreationParams>
    </StampType>
</StampTypes>

The Owner tag assigns it to a player. "0" is you. "1" is Player 2 and so on. 

And this is the spell to summon it:

    <Spells>
    <SpellDef InternalName="DeorcnysseSpell">
        <DisplayName>Summon Deorcnysse</DisplayName>
        <Description>Raises up a darkling city</Description>
        <Image>S_ShadowWorld_Painting.png</Image>
        <IconFG>S_ShadowWorld_Icon.png</IconFG>
        <IconBG>S_ShadowWorld_Icon_BG.png</IconBG>
        <IconColor>32,45,243</IconColor>
        <Cooldown>3</Cooldown>
        <AutoUnlock>1</AutoUnlock>
        <SpellType>Strategic</SpellType>
        <SpellClass>Defensive</SpellClass>
        <SpellSubClass>Other</SpellSubClass>
        <SpellTargetType>EmptyTile</SpellTargetType>
        <TerrainStamp>DeorcnysseStamp</TerrainStamp>
        <AllowStampRotation>1</AllowStampRotation>
        <StampMorphTime>1.0</StampMorphTime>
        <Prereq>
            <Type>AbilityBonusOption</Type>
            <Attribute>Earth2</Attribute>
        </Prereq>
        <GameModifier InternalName="DestroyObjects">
            <ModType>Map</ModType>
            <Attribute>DestroyObjects</Attribute>
        </GameModifier>
        <SpellResourceCost>
            <Resource>Mana</Resource>
            <Amount>1</Amount>
        </SpellResourceCost>
        <ValidTerrainType>Land</ValidTerrainType>
        <AIData AIPersonality="AI_General">
            <AIPriority>5</AIPriority>
        </AIData>
        <HitSoundFX>Spell_Volcano_01</HitSoundFX>
    </SpellDef>
    </Spells>

You don't need the Earth2 to be a prereq, I was just testing things with Gilden. You can create new cityhub types and insert their names into the spell and stamp to create other city types.

I'm not sure how to best make use of these.

At the least we should be able to use them to create new kinds of Pioneers who create specialized kinds of city hubs. For instance the Lord's Manor could be, instead of an improvement in a city, a type of city that only grows to level 2 and only allows a handful of specific buildings. Or, one might be able to strip a "fort" cityhub of it's production and food, give it some defensive and health regeneration bonuses and have an outpost in which you can garrison and heal your troops. It's hard to know what would work best with good gameplay.

Folks with a better understanding of xml could probably make unique and legendary cities, with unique buildings and shop items, spawn during map creation, or even spawn as the reward for a quest.